1. Field of the Invention
The present invention relates to a moisture seal for an F-Type coaxial cable connector.
2. Prior Art
It has been found that the most common reason for degraded TV signals in systems employing outdoor cable or satellite TV cables and/or connections is the ingress of moisture into the coaxial cable through the end connectors. The ingress of moisture into the coaxial cable is primarily due to the pressure changes in small air pockets disposed within the cable during daily and seasonal temperature changes. As the ambient temperature decreases, moist air drawn into the end of the coaxial cable, the moisture eventually corroding the internal shield of the cable. It has therefore become very important to seal the F connectors to the cable and the interconnections between connector and device when used outdoors.
The F connector-to-coaxial cable, and F male-to-female connector interface have four places moisture may enter the interconnection. The points of moisture entry are the interface between: (a) the trailing end of the male connector and the cable; (b) the connector shell and the connector body; (c) the swivel nut and the connector body; and (d) the swivel nut and the F-Type female connector on the device being connected. The foregoing principal sites of water vapor ingress are illustrated in FIG. 6.
Recent advances have been made for connector designs which have effectively sealed the first three moisture ingress locations mentioned above. It is leakage through the last interface (i.e., the interface between the swivel nut on the male F-Type connector and the female F-Type connector on the device being connected) that persists in the art. In accordance with the prior art, the leading end of the internally threaded nut on the male F-Type connector (F connector), which is attached to the cable, is screwed on to the female F connector which has a mating outside thread. The integrity of the interface between the male and female F connectors controls the mechanical and electrical performance of the connection. The thread used on F connectors is a course ⅜-32, specified by the SCTE (Society of Cable Television Engineers) and the EIA (Electronics Industry Association). It is well known that a metal threaded interface of this type cannot be relied upon to restrict the flow of moisture-laden air to the level needed to resist the ingress of such air into the cable over a period of many years exposure. Accordingly, there is a need for a universal sealing device which can protect the interface between the male connector nut and the female shaft from moisture ingress that is operable with the variety of different male connector types available and the variety of female connectors (on devices) with varying lengths, finishes, and bulkheads.
There are presently a few devices and methods used to prevent the ingress of moist air at the male nut-F female interface as illustrated in FIGS. 1-4. These devices only work in selected applications. None of the prior art devices provide an adequate seal between the nut on a male F connector and the threaded shaft of a female F connector which has threads coextensive with the length of the shaft. With reference to FIG. 1, a rubber boot 10 is employed in accordance with the prior art to form a seal between a cable 11 and a ridge 12 that sometime exists on the female F connector 13 mounted on the device 14 being connected to. The rubber boot 10 may keep out some moisture but does not provide a seal that is tight. Further, the device relies on the presence of a sealing ridge 12 on the female connector which is usually absent.
With reference to FIG. 4, air shrink tubing 40 is also employed in the art to provide a seal between the cable 11 and the F-connector 13. Heat shrink tubing cannot be used because the PVC on the coaxial cable jacket will melt. The air shrink tubing 40 presents an inwardly-directed (radial) sealing force but requires a minimal length of the female F connector shaft to be exposed in order to provide a water seal. In addition, the shaft must have a smooth surface. The tubing will not shrink into the threads of the female connector. Therefore this method has a limited application; being operable only for a female F connector having a smooth, unthreaded outer surface on the shaft thereof.
Another sealing technique, though not widely used, is to fill the male connector nut with a silicone grease prior to attachment of the nut to the shaft of the female F connector which will fill the area between threads. This is not recommended due to the difficulty in applying the correct amount of grease as well as the problem of removal and hand cleaning.
The axial compression port seal 20, illustrated in FIGS. 2 and 3, is the most effective sealing device and method employed in the current art. The axial compression port seal 20 consists of a tubular elastic member that slides overt the shaft 21 of the female F connector. When axial pressure from tightening the male nut 22 compresses the elastic device 20, the opposing end of the device 21 exerts an equal force on a bulkhead 23 and thus seals both sides as it compresses. This device 20 and method works well if all sizes are exactly correct for the length of the shaft 21. In practice, with many products being used, this method becomes ineffective. In addition, the axial compression port seal 20 relies on the axial force it exerts on a bulkhead in order to provide a seal. In many devices, this bulkhead does not exist. When an axial compression port seal 20 is used over threads, it cannot exert the needed inward radial force to fill and occlude the thread and pressure seal from its own elasticity. The radial sealing ability of axial compression port seals 20 has been limited due to the need for the installer to slide it over the cylindrical shaft of the female F connector with little effort.
In summary, due to the variety of female connector port lengths, finishes, thread lengths, and the lack of clean, machined bulkheads for axial compression that are currently available on devices being used, it has been almost impossible to achieve moisture ingress protection. Even when a machined bulkhead is available for an axial compression seal, the seal must be sized for the exact length of the female port and male nut so that the proper axial force can be achieved when the male connector is fully screwed in. These three components may be sized correctly to resist moisture ingress for one set of products, but the non-standardization of device dimensions used in the field make these four variables: male nut depth; female shaft length; machined flat bulkhead; and axial rubber seal length combination improbable to find in any particular installation. Therefore, there is a need for a universal port seal that will not require a device to have a proper bulkhead as an abutting end for an axial rubber seal, or extensive non-threaded F-Type female shaft with no cast parting lines as is required for a shrink tube moisture seal.